Artificial spider silk.

"In the spring of 1881 I was a few feet distant from a couple of individuals who were quarreling," George Emery Goodfellow, a physician in Tombstone, Ariz., scribbled in his diary more than a century ago. "They began shooting."

Two bullets pierced the breast of one gunman, who staggered, fired his pistol, and crumpled onto his back.

Examining the body, Goodfellow found that, despite fatal injuries, "not a drop of blood had come from either of the two wounds.

"From the wound in the breast a silk handkerchief protruded," he noted. But when he tugged on the handkerchief, he found the bullet wrapped within it. Evidently, the bullet had torn through the man's clothes, flesh, and bones but had failed to pierce his silk handkerchief, Goodfellow recounted in his "Notes on the Impenetrability of Silk to Bullets."

Fascinated by this, he documented other cases of silk garments halting projectiles-including one incident in which a silk bandanna tied around a man's neck kept a bullet from severing his carotid artery. "The life of this man was, presumably, saved by the handkerchief," Goodfellow wrote.

The strength, toughness, and elasticity of silk continue to intrigue scientists, who wonder what gives this natural material its unusual qualities.

Finer than human hair, lighter than cotton, and-ounce for ounce-stronger than steel, silk tantalizes materials researchers seeking to duplicate its properties or synthesize it for large-scale production.

Visions of wear-resistant shoes and clothes; stronger ropes, nets, seatbelts, and parachutes; and rustfree panels and bumpers for automobiles all dance through researchers' minds. So do improved sutures and bandages, artificial tendons and ligaments, and supports for weakened blood vessels. Soldiers and police long for bulletproof vests of spider silk.

While many insects secrete silks of varying quality, the dragline silk of the golden orb-weaving spider, Nephila clavipes, has attracted the most scientific attention. Researchers marvel at its high tensile strength and ability to stretch without snapping. It is tougher, stretchier, and more waterproof than the silkworm's strands used today in fine garments.

Spider dragline silk "exhibits a combination of strength and toughness unmatched by high-performance synthetic fibers," says David A. Tirrell, a materials scientist at the University of Massachusetts at Amherst. Even though it's lighter, dragline silk has proven itself in many ways superior to Kevlar, the strongest synthetic polymer, agrees Lynn W. Jelinski, a biophysicist at Cornell University. "The question is whether we can use our understanding of dragline silk proteins to produce a bio-inspired material."

Dragline silk provides a frame for spiderwebs and enables a dangling spider to plummet down and nab its prey. Because the orb weaver's survival depends on dragline silk, some 400 million years' of evolution have fine-tuned a "remarkably tough and versatile material," says John M. Gosline, a biologist at the University of British Columbia in Vancouver.

Now, several research groups are vying to spin the first artificial spider silk, a feat that requires a three-pronged approach, says Jelinski. One must determine the fiber's molecular architecture, understand the genes that yield silk proteins, and learn how to spin the raw material into threads. Working with chemist Alexandra H. Simmons and physicist Carl A. Michal, both at Cornell, Jelinski proposed in the Jan. 5 Science a model to explain dragline silk's strength and elasticity.

Scientists had known for years that, of the 20 natural amino acids, only 7-alanine and glycine, with lesser amounts of glutamine, leucine, arginine, tyrosine, and serine-serve as silk's primary constituents. Their exact sequences and structural relationships, however, had remained elusive. Jelinski and her colleagues have used nuclear magnetic resonance (NMR) to show how the natural silk fiber's main components hang together. The fiber is made up of two alanine-rich proteins embedded in a jellylike polymer. Jelinski's group found that the crystalline structure of one of the proteins is highly ordered and the structure of the other is less ordered. These proteins stick to the glycine-rich polymer, which makes up about 70 percent of the material. Based on the NMR studies, Jelinski argues that dragline silk's strength and elasticity derive from a blend of ordered and disordered components. The silk's amorphous polymer, resembling a "tangle of cooked spaghetti," makes the fiber elastic, while the two types of protein give it toughness.

Moreover, Jelinski holds that the synthetic silk of the future shouldn't be "too regular" in its molecular patterning. "Nature's randomness," she says, "would give the material extra strength."

For the spider dragline silk, scientists believe they have identified the entire genetic sequence, which measures more than 22,000 base pairs. But they disagree about how much of that sequence needs to be cloned to make proteins good enough to spin into top-quality synthetic threads.

Long stretches in the sequence may be inconsequential to the material itself, functioning as regulatory genes for the spider's own purposes. Some scientists believe that as few as 300 base pairs may suffice to make a good silk, but others hold that several thousand or even the entire sequence is needed. Randolph V. Lewis, a molecular biologist at the University of Wyoming in Laramie, has identified genes for dragline silk's two main proteins. His team recently cloned portions of those genes and implanted them in the bacterium Escherichia coli. He has coaxed the bacterium into producing silk protein in solution, which he squeezes through a fine tube to make synthetic silk fibers.

"I think soon we'll be able to make a close analog of spider silk," says Lewis. "Will it be identical to silk? Probably not. But it may still be an excellent fiber."

Lewis says he doubts there's anything "magical" about the way spiders spin silk. "They're not even good at making fibers," he says. "Spiders vary the silk's consistency too much. A manufacturer wouldn't tolerate so much variation."

Ideally, he wants to do more than just replicate natural silk strands. "I want to control silk's properties," Lewis says.

At the U.S. Army's Natick (Mass.) Research, Development & Engineering Center, David L. Kaplan and his colleagues also have set up a program to fabricate spider silk.

Using techniques similar to those of Lewis, Kaplan's team has identified what they believe are the critical portions of the dragline silk genes, then fashioned polymer fibers based on those several hundred base pairs. They're banking on the idea that they don't need to replicate the entire set of genes.

Rather, by focusing on just the portions of proteins believed to make silk tough, they think they can produce silklike threads.

Plants and fungi, as well as bacteria, could serve as hosts for artificial genes. Kaplan says that if a robust plant could express a dragline silk gene, perhaps silk proteins could be harvested in vast quantities, processed into a liquid polymer, and spun in factories.

"Now we're spinning silk fibers from the synthetic proteins," he says. The Army's interest in artificial silk lies in making durable and protective clothing, parachutes, and war paraphernalia-perhaps even bulletproof vests to replace existing Kevlar ones.

"We want a biologically inspired synthetic fiber with many uses," he adds. "It should be as tough as natural silk but easier and cheaper to make."

Cloning the entire silk protein is not necessary, agrees John P. O'Brien, a chemist at DuPont Co. in Wilmington, Del. "We think we can mimic most of natural silk's properties with much simpler polymers and produce them large-scale.

"Silk has a lot in common with reinforced rubber," he adds. "This allows us to use theories of rubber elasticity to design the synthetic fiber's architecture."

To reduce the length and complexity of the synthetic protein, DuPont chemist Stephen R. Fahnestock says his group has homed in on four short amino acid sequences from one of the two major proteins. By implanting a synthetic gene for those sequences, his team has coaxed bacteria and yeast into producing a novel protein, which DuPont is spinning like conventional polymers into fibers.

"They're not quite like natural spider silk," says O'Brien, "But they're still good when woven into multifilament yarns."

Kenn H. Gardner, a biophysicist at DuPont, points out that spider silk, both the natural and new synthetic versions, is essentially a form of nylon. "That's our business," he says.

"What's particularly interesting to us is the way these organisms make silk nylons in environmentally benign ways," O'Brien says. "They process proteins from water-based solutions, without using petroleum products or organic solvents. From a manufacturing point of view, this is very attractive." Given the "consumer love affair with natural fibers," he adds, "we want to offer substitutes for natural fibers that are free of associated problems, such as poor wash-wear performance, stretching, wrinkling, and shrinkage. "Ideally, we're aiming for a better-than-natural alternative fiber."

Taking a different approach, chemists Glenn R. Elion of Plant Cell Technologies in Chatham, Mass., and Richard M. Basel of Lebensmittel Consulting in Fostoria, Ohio, are going for the entire silk gene. They're working with what they believe is the full dragline silk gene sequence. Using a technique for which they have a patent pending, they have moved that sequence of 22,000 base pairs into bacteria and obtained enough raw spider silk to begin spinning fibers, they claim.

Elion says that his group also is aiming to insert the whole dragline silk gene into high-protein plants, like soy, to produce large yields of silk protein more efficiently.

The researchers are also attempting to alter the silk's color, he says. Spun into threads, natural dragline silk glistens in glorious golden tones. By tinkering with regulatory genes that spiders use for camouflaging their silk, Elion believes that he may be able to generate other colors.

Many species of spiders produce up to seven kinds of silk, with different strength, flexibility, stickiness, and translucence. "Spiders adjust their silk's properties by expressing different genes in different glands," says Gosline. "We're still not quite sure how they do it." In his efforts to quantify the mechanical properties of different silks, Gosline is toying with spider silk genes to find out how to fine-tune the material's quality for specific applications.

In addition to the genes for the dragline silk proteins of the golden orb weaver, Gosline's group has gone after four related genes in another spider, Araneus diadematus, that produces an unusual silklike protein. Gosline's team is manipulating these genes to figure out how to vary the silk's qualities. "We're finding that different genes produce proteins containing differing amounts of crystalline material," Gosline says. "Somehow, spiders use this to modulate silk's properties."

Gosline says that he's searching for the rules governing silk's structure. "If we can change silk's properties in predictable ways, then we can use those rules to tailor its production to specific applications.

"Maybe through genetic engineering we can make silk proteins that have never gotten expressed through natural evolution," he adds. The practical and economic potential of generating artificial spider silk sings a siren's song to biotechnologists. Globally, as much as 60 percent of the threads used to weave clothing come from natural fibers, including cotton, wool, and silk. "We're talking about billions of dollars," says Elion. "This is a major market."

"Bio-inspired materials are providing a new frontier for the fiber business," Jelinski says. "Someone's going to hit a home run in this field. But I'm not sure yet who it will be."

Electron Pairs and Waves Tackling the puzzle of high-temperature superconductivity

The detectives have been hard at work gathering clues, building theories, and sketching portraits of the perpetrator. Yet, after years of dogged effort, no satisfying resolution of the mystery appears in sight. The culprit continues to elude the supersleuths.

In the case of high-temperature superconductors, the missing factor-the elusive culprit-is the mechanism that allows certain copper oxide ceramic compounds to conduct electricity without resistance. How do electrons manage to flow effortlessly through the ceramic compounds at temperatures as high as 135 kelvins?

The discovery of these unusual superconductors in 1986 took researchers by surprise. No one had ever suspected that such ceramic compounds could become superconductors or, indeed, that any material could be a superconductor at a temperature higher than about 20 kelvins, the highest temperature at which certain metals can behave as superconductors.

Besides their high transition points, these ceramic compounds have several features that set them apart from previously known superconductors. At temperatures at which the ceramic materials are not superconductors, they exhibit behavior quite unlike ordinary metals. Made up of sheets of copper and oxygen atoms sandwiched between layers of other atoms, they conduct electric current better in some directions than in others, and minor chemical changes transform them into electrical insulators with striking magnetic characteristics.

For most metals, including aluminum, scientists can apply quantum theory to explain why mobile electrons act as if they were free. At least one electron from each atom moves about independently, collectively forming a so-called Fermi liquid. On this basis, the scientists can make predictions about the metals' characteristics and behavior.

"The new materials, however, seem to require new principles," says Sudip Chakravarty of the University of California, Los Angeles.

"In these materials, electron motions are so strongly modified by the repulsive forces exerted by their neighbors that we can no longer approximate their motions as being independent," says Piers Coleman of Rutgers University in Piscataway, N.J.

Writing in the December 1995 Physics World, he contends that high-temperature superconductivity is just one example of "the unexpected consequences of collective behavior in vast assemblies of interacting particles." Hence, detailed investigations of electron activity within such compounds provide important clues that may ultimately lead to a theory that accounts for their distinctive character both as metals and as superconductors.

Pinpointing the mechanism of high-temperature superconductivity would also make it possible for researchers to tailor new materials to specific purposes.

Conceivably, they could push superconducting transition temperatures significantly higher than those presently achievable-perhaps even to room temperature (about 300 kelvins).

"It's a fascinating puzzle," says M. Brian Maple of the University of California, San Diego. "These high-temperature superconductors were completely unexpected in the beginning, and they are now a rich reservoir of interesting phenomena to study."

The theoretical starting point is a model, proposed in 1957 by John Bardeen, Leon N. Cooper, and J. Robert Shrieffer to account for superconductivity in metals such as aluminum and zinc at temperatures close to absolute zero. This theory, called BCS after its originators, is based on the notion that current-carrying electrons can overcome their mutual repulsion and pair up in ways that allow them to pass unhindered through the host material. In conventional, low-temperature superconductors, this pairing is facilitated by vibrations of the crystal lattice through which the electrons travel. A moving electron induces slight displacements in the positions of positively charged ions along its path, causing a ripple-like effect. A second electron can get caught in this ripple, and it can end up traveling as if its motion were coordinated with that of the first.

Because the electrons of a pair have opposite spin, as a unit they can move through material without resistance. Quantum mechanics describes the pair by means of a single wave function, which mathematically specifies a probability distribution showing where the two electrons are most likely to be. In this case, the wave function is spherical, indicating that the electron pairs have an equal chance of moving in any direction. Such a pairing is said to display s-wave symmetry.

When the copper oxide superconductors were discovered, researchers tried to apply the same theory, beginning with the notion that the current-carrying electrons must move in pairs. They quickly realized that in these materials, lattice vibrations alone aren't strong enough to maintain such pairing at the high superconducting transition temperatures observed in the copper oxides. Theorists have since proposed a number of different mechanisms that they believe could produce the necessary electron pairing and permit superconductivity at elevated temperatures. Some of these theories invoke magnetic spin interactions between electrons and copper ions in the copper oxide layers, while others rely on such effects as electron tunneling between sheets of copper and oxygen atoms in these compounds.

To shed some light on the mechanism responsible for electron pairing, researchers have performed a wide range of experiments on high-temperature superconductors. One particular series has focused on whether electron pairs in these compounds fit an s-wave pattern or an alternative symmetry known as d-wave.

In the d-wave case, the wave function of the electron pair resembles a four-leaf clover, with lobes aligned along the crystal axes of the material (see illustration below). This shape means that the probability of electron travel is higher in some directions than in others. Moreover, electrons moving along one axis are out of step, or phase, with electrons moving along the axis at right angles to it.

One approach is to check whether the energy needed to break up an electron pair depends on its direction of travel. That can be done by bombarding a crystal's surface with high-energy photons. These photons knock electrons out of the material, and researchers can measure the energies of the ejected particles.

Last year, Zhi-Xun Shen of Stanford University and his collaborators used this technique, known as photoemission spectroscopy, to determine the binding force between paired electrons in six high-temperature superconductors, including yttrium barium copper oxide. They measured the energy and direction of electrons emitted by each material, and they found that the binding force did vary, falling to nearly zero along certain directions relative to the material's crystal lattice (SN: 2/11/95, p. 88).

These results support the idea that electron pairing is characterized by d-wave symmetry.

Other groups have focused on detecting the changes in phase of the electron pair motion that should occur with d-wave pairing. In 1994, John R. Kirtley, Chang C. Tsuei, and their coworkers at the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y., used an ingenious arrangement to probe these effects in yttrium barium copper oxide (SN: 4/2/94, p. 213). Their results were consistent with d-wave pairing.

Now, the IBM team reports in the Jan. 19 Science that a new experiment involving thin films of another superconductor, thallium barium copper oxide, produces the same result.

These and several other key experiments over the last few years all strongly indicate that d-wave rather than s-wave interactions predominate in high-temperature superconductors.

"So the question in my mind is no longer whether it's d-wave," says Douglas J. Scalapino of the University of California, Santa Barbara. "Instead, the question becomes: If it's d-wave, what does that tell us?"

The trouble is that a variety of quantum mechanical effects can lead to d-wave symmetry. No single answer automatically emerges to pin down what causes the pairing.

Nonetheless, these results are encouraging for theorists, like Scalapino, who have proposed that pairing results from the way electrons interact with fluctuations in the spins of neighboring copper ions in the crystal lattice. The presence of such interactions favors electron pairs with d-wave symmetry, they argue, and the experimental results bolster this argument.

At the same time, detailed theoretical calculations have so far failed to demonstrate that spin fluctuations are sufficient to initiate superconductivity at the temperatures observed in the copper oxides-unless the calculations are based on unrealistic assumptions about the characteristics of the materials.

"My sense is that we are on the right track," Scalapino says. "But there may be a piece of this puzzle that we don't understand yet, something missing that would provide the necessary [interaction] strength.

"On the other hand, these ideas could be incorrect, as some people believe, and we need something else," he concedes.

In addition to problems with the calculations, there's one other discordant note. Experiments by San Diego's Robert C. Dynes and his colleagues have consistently provided evidence of an s-wave contribution, at least in yttrium barium copper oxide. The researchers observe electron tunneling between the material's layers, a direction that s-wave, but not d-wave, symmetry would allow.

"It's possible that we have a situation in which there is both s-wave and d-wave character in the superconducting state of yttrium barium copper oxide," Maple says.

Alternatively, "there may be some subtle features that none of us fully appreciate which could lead to one conclusion or the other," he adds. "One simply needs to do more experiments on more materials, doing a variety of measurements."

It's even possible that no single mechanism applies to all the different high-temperature superconductors now identified. At least one, neodymium cerium copper oxide, is already known to have electrical properties that distinguish it from the others.

"I don't think the story is over yet," says Victor J. Emery of the Brookhaven National Laboratory in Upton, N.Y. "These are complicated materials, and one should try to take into account all the experiments.

"This whole business of [high-temperature] superconductivity has focused attention on a lot of scientific issues that are too interesting to be brushed away," he adds.

How to deal with the complexities has divided the physics community. "One group believes that BCS theory can be extended to understand these systems without any major revision of the way we think about metals," Coleman comments. "An opposing school identifies many of the unusual properties of the normal [metal] state as evidence for new physics that is intimately related to high-temperature superconductivity."

Philip W. Anderson of Princeton University is among those in the latter category. He insists that it's time to rethink not only superconductivity theory but also the standard theory of electron behavior in metals.

In Anderson's view, high-temperature superconductivity is just one example of a phenomenon for which this picture may be misleading. Mechanisms such as pairing are too simple to account for this behavior, and repulsive interactions between electrons must also be included.

"What is clear is that the 2 decades or more of efforts to fit all these phenomena into a Fermi liquid description are a catalog of failure," Anderson argues in the December 1995 Physics World, "and it is time we opened our minds to new ways of thinking."

At this stage, the detectives can't even be sure they're on the trail of the culprit. It may be heavily disguised or an as-yet-unsuspected party.